Tractors and other off-road work vehicles in the agricultural, mining and construction industries have typically operated with manual steering by the operator. Recent changes in control systems and the development of satellite navigation systems (e.g global positioning systems or GPS) have allowed tractors to operate in a semi-automatic or fully automatic steering mode. Combining satellite navigation and ground-based navigation input signals regarding vehicle position and speed with a sophisticated on-board vehicle electronic control system allow the tractor to steer itself with a high degree of accuracy when traversing terrain.
To provide this control, the prior art teaches using satellite navigation system information by an on-board vehicle electronic control system to accurately determine and control a vehicle's position while operating in the field. The operator will typically enter the planned route of the tractor, or let the control system determine the most efficient route. The control methods are well known in the art, and may involve multiple position transmitters or receivers, with various signals signifying location and speed. However these methods do not control a towed implement accurately, as the towed implement does not follow the same path as the towing vehicle.
Tractors do not generally to agricultural work directly. Instead, they tow implements that have several ground engaging tools. If the tractor guides itself with a high degree of accuracy, and the implement is rigidly attached to tractor, the implements can follow the ground and their tools can engage the ground with a high degree of accuracy (assuming the tractor is guided with a high degree of accuracy). Not all implements are rigidly attached to the tractor, however. Many are pivotally attached to tractor, like a trailer towed behind an automobile. Because of this pivotal coupling with respect to the tractor, controlling the tractor's position with a high degree of accuracy does not guarantee that the implement position is going to be similarly controlled.
To solve this problem, the prior art teaches the addition of one or more GPS receivers on the towed implement. This method gives a more accurate location for the towed implement, but requires a GPS for each implement. Typically agricultural operations will require several different towed implements to affect the field during the growing cycle of a crop. This would require several GPS receivers, or the removal and reattachment of GPS receivers to each successive implement, in order to allow each towed implement to be tracked accurately according to a planned implement path in the field.
Another proposed solution has been to provide the towed implement with a discrete control system that lets the implement determine its position and steer itself with respect to the tractor. The drawback of the system is the addition of a discrete control system as well as steering actuators and position sensors for every implement.
What is needed is a more accurate and inexpensive method of determining and controlling the path of a towed implement, using the steering actuator of the tractor itself. What is also needed is a relative position sensor that can be used with a variety of implements, and does not need to be removed and reinstalled, or duplicated for each towed implement. What is further needed is a guidance system that allows the operator to input the geometry of a towed implement, and then uses the vehicle position and towed implement geometry and relative position to accurately control the implement path by actuating the tractor steering mechanism.